1. Field of the Invention
This invention relates to a storage container for storing and keeping a radioactive substance that involves generation of heat, a refilling system for loading into and unloading a metallic closed container or a so-called canister, stored with the radioactive substance, from the storage container, and a refilling method.
2. Description of the Related Art
Highly radioactive substances, represented by used fuels from nuclear reactors, are reprocessed to recover useful substances that can be reused as fuels such as plutonium. These used fuels are hermetically sealed as they are stored for reprocessing. A dry method using a storage container such as a cask is regarded as a noticeable storage method for these highly radioactive substances. There are casks of various constructions that are used for the dry method. A concrete cask, a concrete structure that shields a used fuel, attracts special attention owing to its low cost. Concrete has the advantage of necessary strength for a structure and the like.
The concrete cask of this type is provided with a tubular concrete container that is closed at the top and bottom portions, and a metallic closed container or a so-called canister, in the form of a tube having a used fuel sealed therein, is stored in the concrete container.
In general, a canister is heated to a high temperature of about 200xc2x0 C. by decay heat from a used fuel. Therefore, the concrete cask is provided with a heat removing structure for removing the decay heat from the used fuel. More specifically, an annular gap that serves as a cooling air passage is defined between the inner peripheral surface of the concrete container and the outer peripheral surface of the canister. The peripheral edge portion of the lower end of the concrete container is provided with intake vents, and the peripheral edge portion of the upper end of the container with exhaust vents. As the open air as cooling air that is introduced into the concrete container through the intake vents is run in the cooling air passage for natural convection and discharged through the exhaust vents, the canister and the concrete cask are relieved of heat and cooled.
In the concrete cask constructed in this manner, the aforethe heat removing structure, concrete layer, and canister ensure the cooling of the used fuel, shielding of radiation, and hermetic sealing of the used fuel, respectively. The concrete cask must keep a highly radioactive substance safely and steadily for a long time, and is expected to have prolonged high radiation shielding performance.
On the other hand, the following processes are carried out to load the radioactive substance into and unload it from the concrete cask of this type.
First a used fuel from a nuclear reactor, as the radioactive substance, is stored and hermetically sealed in a canister in a storage pool of a nuclear power plant or the like, for example. After the canister is loaded into a transportation container or a so-called transportation cask, it is transported in a truck or the like to a storage facility. In this storage facility, the transported canister is drawn out of the transportation cask and loaded into a prepared concrete cask. The canister is stored and stocked in the concrete cask for a given time.
After it is thus stocked for the given time, the canister is unloaded from the concrete cask, loaded again into the transportation cask, and transported in a truck or the like to a reprocessing facility.
Normally, the concrete cask is set upright when the canister is loaded into and unloaded from the concrete cask or during canister refilling operations. The canister is suspended from above as it is loaded into the concrete cask or unloaded from the concrete cask through a top opening of the concrete cask.
In the canister refilling operation described above, however, the suspended canister must be supposed to fall from some cause, in some cases. If the canister situated right over the concrete cask or partially inserted in the canister falls during the refilling operation, for example, the canister may be supposed to run against the bottom wall of the concrete cask through the interior of the concrete cask and be broken by the resulting impact.
In order to secure the sealing and shielding properties of the canister and the concrete cask against radiation and further improve the reliability and safety, therefore, a countermeasure is needed to prevent breakage of the canister attributable to the aforethe falling.
This invention has been made in consideration of these circumstances, and its object is to provide a storage container, a storage container refilling system, and a refilling method, in which breakage of a closed container can be prevented if the closed container falls during refilling operation for the closed container or the like.
In order to achieve the above object, a storage container according to an aspect of this invention comprises: a container body having a storage portion for storing a closed container, having a radioactive substance sealed therein, and a top opening through which the closed container is loaded into and unloaded from the storage portion; a lid closing the top opening of the container body; a heat removing portion having intake vents in a bottom portion of the container body, exhaust vents in a top portion of the container body, and a cooling air passage defined between an inner surface of the storage portion and an outer surface of the closed container stored in the storage portion, the heat removing portion being configured to cause air introduced into the container body through the intake vents to flow into the cooling air passage so as to remove heat from the radioactive substance and discharge the air through the exhaust vents; closing members configured to close the intake vents and exhaust vents as the closed container is loaded into and unloaded from the storage portion; and a speed reducing portion provided at the top opening of the container body and having a through hole through which the closed container can be passed, the through hole having a dimension smaller than the dimension of the storage portion and larger than the outside dimension of the closed container.
In loading into or unloading the closed container from the storage portion, according to the storage container with this construction, the closed container is loaded or unloaded through the through hole of the speed reducing portion. Since the this through hole has the dimension smaller than the dimension of the storage portion and larger than the outside dimension of the closed container, the gap between the outer surface of the closed container and the through hole that is formed as the closed container passes through the through hole can be made narrower than the gap between the closed container and the inner surface of the storage portion with the closed container set in the storage portion. By closing the intake vents and exhaust vents by means of the closing members, at the same time, air in the storage portion can be discharged to the outside only through the narrow gap between the outer surface of the closed container and the through hole.
If the closed container falls during refilling operations for the closed container, therefore, air that passes through the gap between the outer surface of the closed container and the through hole has high resistance and acts as an air damper, so that the falling speed of the closed container can be reduced considerably. Thus, if the closed container falls owing to any accident or the like, impact that acts on the closed container can be lowered to prevent breakage of the closed container.
Further, a storage container refilling system according to an aspect of this invention is a storage container refilling system for loading into and unloading a closed container having a radioactive substance sealed therein from a storage container, which comprises a container body having a storage portion for storing the closed container and a top opening through which the closed container is loaded into and unloaded from the storage portion, a lid closing the top opening of the container body, and a heat removing portion having intake vents in a bottom portion of the container body, exhaust vents in the top portion of the container body, and a cooling air passage defined between an inner surface of the storage portion and an outer surface of the closed container stored in the storage portion, the heat removing portion being configured to cause air introduced into the container body through the intake vents to flow into the cooling air passage so as to remove heat from the radioactive substance, and discharge the air through the exhaust vents, the storage container refilling system comprising:
a reduction cylinder having an outside dimension smaller than the dimension of the storage portion and an inside dimension larger than the dimension of the closed container and capable of being loaded into the storage portion through the top opening; a first lift mechanism configured to raise and lower the reduction cylinder between a down position in which the reduction cylinder is set in the storage portion of the container body and an up position such that the reduction cylinder is drawn out of the storage portion; and a second lift mechanism having a holding portion configured to hold one end portion of the closed container and to raise and lower the closed container with respect to the container body, the second lift mechanism being configured to load into and unload the closed container from the storage portion of the container body with the reduction cylinder set in the storage portion.
Furthermore, a storage container refilling method according to an aspect of this invention is a storage container refilling method for loading into and unloading a closed container having a radioactive substance sealed therein from a storage container, which comprises a container body having a storage portion for storing the closed container and a top opening through which the closed container is loaded into and unloaded from the storage portion, a lid closing the top opening of the container body, and a heat removing portion having intake vents in the bottom portion of the container body, exhaust vents in the top portion of the container body, and a cooling air passage defined between the inner surface of the storage portion and the outer surface of the closed container stored in the storage portion, the heat removing portion being configured to cause air introduced into the container body through the intake vents to flow into the cooling air passage so as to remove heat from the radioactive substance, and discharge the air through the exhaust vents, the storage container refilling method comprising: loading a reduction cylinder, having an outside dimension smaller than the dimension of the storage portion and an inside dimension larger than the dimension of the closed container, into the storage portion through the top opening of the storage container; inserting the closed container into the reduction cylinder from above through the top opening of the reduction cylinder, thereby locating the closed container in the storage portion; then pulling up the reduction cylinder from the storage portion; and unloading the closed container from the storage portion in a manner such that the closed container is pulled up from the storage portion after the reduction cylinder is loaded into the storage portion through the top opening of the storage container and located outside the closed container.
According to the refilling system and the refilling method arranged in this manner, the reduction cylinder is previously set in the storage portion of the storage container before the closed container is loaded into and unloaded from the storage portion. When the reduction cylinder is set in the storage portion, a space having a dimension smaller than that of the storage portion is defined in the storage portion by the reduction cylinder. Thus, the gap between the outer surface of the closed container that passes through the reduction cylinder and the inner surface of the reduction cylinder is narrower than the gap between the closed container set in the storage portion and the inner surface of the storage portion. Air in the reduction cylinder is discharged to the outside through this narrow gap only. If the closed container falls during the refilling operation for the closed container, therefore, the air that passes through the gap between the inner surface of the reduction cylinder and the outer surface of the closed container has high resistance and acts as an air damper, so that the falling speed of the closed container can be reduced considerably. Thus, if the closed container falls owing to any accident or the like, the impact that acts on the closed container can be lowered to prevent breakage of the closed container.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.